How can we practice fasting as worship in spirit?

The Church Fathers linked fasting to worship in spirit and truth; namely, prayer, contemplation on the word of God, prostrations (metanoias), almsgiving, and the partaking of the holy mysteries.

St. Basil the Great says, “may the fast with which the Jews were threatened not come upon us: Behold the days are coming, says the Lord, and I will send a famine upon this land, not a famine of bread, nor a thirst for water, but a famine of hearing the word of the Lord (Amos 8:11). It was for this reason that the Just Judge sent it, because he saw their mind was wasting away from a lack of the nourishment that comes from the doctrines of truth, yet their external man was growing enormously fat and obese. And so, in the coming days the Holy Spirit will give you a feast at both the morning and evening festivities. No one should willingly absent themselves form this spiritual banquet. Let all of us share in the sober cup. It has been prepared by Wisdom, who hands it to us equally, to the extent that each has the capacity to drink it. For she has prepared her cup and slaughtered her beasts (Prov. 9:2), that is, the food for the perfect, for those who have their faculties trained by practice to distinguish good from evil (Heb. 5:14). Filled with such an abundance, may we be found worthy too of the joy experienced in the bridegroom’s chamber, in Christ Jesus our Lord.”⁴³⁵

The scholar Tertullian says, “They who are about to enter baptism ought to pray with repeated prayers, fasts, prostrations, and vigils all the night through, and with the confession of all bygone sins.”⁴³⁶

St. Ambrose says, “Blessed is he that knows how to be full in Christ. Not corporal, but spiritual, is that satiety which knowledge brings about.”⁴³⁷

St. Isaac the Syrian links fasting to prayer when he likens fasting to a live coal which, if no incense of prayer is sprinkled on it, will produce a choking smoke rather a sweet aroma of incense. Tertullian says that we should “feed prayers on fastings.”⁴³⁸

434 St. Basil of Caesarea, On Fasting and Feasts 1.7 (PP 50:64).

435 St. Basil of Caesarea, On Fasting and Feasts 2.8 (PP 50:81).

436 Tertullian, On Baptism 20 (ANF 3:678).

437 St. Ambrose of Milan, Duties of the Clergy 2.17.92 (NPNF II/10:57).

438 Tertullian, On Repentance 9 (ANF 3:664).

St. Augustine links fasting with brotherly love saying, “In this question, however, of fasting or not fasting on the seventh day, nothing appears to me more safe and conducive to peace than the Apostle's rule: ‘Let not him who eats despise him who does not eat, and let not him who does not eat judge him who eats’ (Rom. 14:3) ‘for neither if we eat are we the better, nor if we do not eat are we the worse’ (1 Cor. 8:8). Our fellowship with those among whom we live, and along with whom we live in God, being preserved undisturbed by these things.”⁴³⁹